Filter apparatus

ABSTRACT

The invention relates to a filter apparatus ( 10 ) for cleaning functional fluids, especially selected from phosphate esters, phosphate ester-based oils, mineral oils and phenols, comprising a filter housing ( 12 ) into which a filter bed ( 14 ) has been introduced, especially consisting of a mixture of anionic and cationic ion exchange resin, and comprising at least one filter element ( 16 ) which, designed as a flow guide device, enables the diversion of the fluid from a radial to an axial flow direction ( 18 ) or vice versa.

The invention relates to a filter apparatus for cleaning of functional fluids, especially chosen from among phosphate esters, phosphate-ester based oils, mineral oils, and phenols, having a filter housing into which a filter bed is inserted, especially consisting of a mixture of anionic and cationic ion-exchange resins.

These functional fluids are used in hydraulic systems, such as, for example, gas turbines and steam turbines, and in the turbine control as lubricants or also as hydraulic media. Often, in addition to needing to be suitable as lubricant media or hydraulic media, it is furthermore required that the fluids be poorly flammable. To achieve proper operation of the respective hydraulic system, it is necessary for the fluid used, during the entire time that it circulates, for example, in a fluid circuit of the hydraulic system, to have a very high quality, and in particular impurities of any kind which accompany especially the degeneration of the fluid in use are allowed only up to a maximum amount, which must generally be set very low. Therefore, in hydraulic systems which do not provide cleaning of the fluid, when a maximum allowable contamination is reached, the complete replacement of the used fluid by a new fluid is necessary. This is, on the one hand, very labor-intensive in most hydraulic systems, and, on the other hand, the indicated fluids are very expensive, especially when they are endowed with special properties such as being poorly flammable and the like. This prompts a legitimate interest in prolonging the service life of the fluid used, and the cleaning of the fluid in particular lends itself for accomplishing this purpose.

Filter apparatus having a filter bed of anionic and cationic ion-exchange resins (U.S. Pat. No. 3,708,508 and U.S. Pat. No. 4,741,857) are used to remove corrosive acids from functional fluids, especially poorly flammable, phosphate ester-based hydraulic fluids (HFD-R). Aging of these fluids is due to hydrolysis, in which the decomposition products are, for instance, corrosive fluids. The neutralization number N_(z), in English also referred to as “TAN” (Total Acid Number), and the content of metal soaps can be reduced by special ion-exchange resins. In addition to this cleaning of the fluids, continuous dewatering of the fluid is often carried out as well. The desired value of the neutralization number of the cleaned fluid is N_(z)≦0.1 mg KOH/g_(fluid), and, for practical use of the fluids in hydraulic systems, values of the neutralization number N_(z) of up to 1 mg KOH per gram of fluid can be easily achieved.

EP 0 696 311 B1 discloses a method for treatment of a nonaqueous functional fluid, selected from among phosphate esters, mineral oils, and carboxylate esters, in which a portion of the fluid quantity makes contact with an anionic ion-exchange resin in the presence of enough water so that the acid content of the fluid can be reduced. Then, in the known solution, the removal of water from the fluid by vacuum dewatering and finally the combination of the treated fluid with the bulk of the remaining fluid take place (in situ).

U.S. Pat. No. 5,661,117 B1 discloses a method for regeneration of phosphate ester lubricating fluids in which the lubricating fluid circulates in a mechanical system and is contaminated with metallic material and phosphorus-based acids and in which cleaning leads to a new fluid quality with a TAN (Total Acid Number) of 0.03, where said method consists essentially of the following steps:

-   -   Preparation of a source of a phosphate ester lubricant which         circulates in a mechanical apparatus and which has at least one         metal from the group consisting of aluminum, chromium, tin,         iron, sodium, calcium, magnesium, silicon, and phosphorus-based         acids;     -   Preparation of a source of anionic ion-exchange resin in a         cartridge in fluid communication with the apparatus, with the         resin having a liquid content of at least roughly 50%;     -   Introduction of the phosphate ester lubricating fluid         contaminated with the metals and the acids for purposes of         contact with the resin; and     -   Removal of the metallic compounds and the acids, using the resin         by replacing the filter with a periodicity of up to 27 months,         when there is a rise in the TAN of roughly 0.07 in order to make         available a reusable lubricating fluid which is essentially free         of impurities in new quality and which has a Total Acid Number         of 0.03.

Proceeding from this prior art, the object of the invention is to make available a filter apparatus for cleaning of functional fluids, which in addition to a compact structure ensures high reliability and also simple manageability in practical use. The cleaning of the functional fluid in the filter apparatus according to the invention is intended to lead to a high-quality fluid with the best possible use being made of the filter bed. Furthermore, the filter apparatus is also to be able to be easily retrofitted into existing hydraulic systems.

This object is achieved according to the invention by a filter apparatus having the features specified in claim 1 in its entirety.

Generally, the filter apparatus will be connected in the bypass flow to an existing tank of a hydraulic system. An exactly matched fluid volumetric flow passes through a filter bed of preferably anionic and cationic ion-exchange resin in transverse flow velocity. During this flow, the acids, metallic soaps, and other basic media contained in the fluid and particulate dirt, which may be present, are removed. In this way, an improvement of the quality of the functional fluid is achieved, for instance, by the lowering of the neutralization number.

In that in the filter apparatus, in addition to one filter bed, there is at least one filter element which, designed as a flow guide device, allows a diversion of the fluid from a radial into an axial flow direction or vice versa, a compact unit is formed. In that at least one filter element, designed as a flow guide device, diverts the flow direction of the fluid, the respective structural conditions of the hydraulic system to which the filter apparatus is to be connected can be taken into account in order to implement the fluid flow directions which are best for optimum cleaning success in this way. For example, a large region around the filter apparatus can be included for particle removal by a radial incident flow on the filter apparatus, while, for example, an axial outflow of the fluid from the filter apparatus is advantageous for connecting pipes or a pump for low-cavitation relaying of the fluid. The solution according to the invention either can be executed with a housing made in one piece, or there can be a multi-part structure of the filter apparatus in which one or more filter elements are detachably fixed on the rest of the apparatus housing.

In one advantageous exemplary embodiment, viewed in the flow direction of the fluid, at least one flow guide device made as a filter element is located upstream of the filter bed and can thus undertake a first particle removal from the fluid especially in the manner of a prefilter and therefore can protect an ion-exchange resin which has been introduced as a filter bed against the ingress of a portion of the contaminants of the fluid. Viewed in the flow direction of the fluid, in another especially preferred exemplary embodiment there is another flow guide device made as a filter element downstream of the filter bed which, in the manner of a protective filter, is used for the re-cleaning of the fluid and the protection of the hydraulic system against possible discharge of the ion-exchange resin. In particular, combining the prefilter upstream of the filter bed and the protective filter downstream of the filter bed into one filter apparatus leads to a very compact structure. The filter apparatus according to the invention is made in the manner of a manageable cartridge which can also be easily retrofitted into existing hydraulic systems within the framework of a retrofit.

In one advantageous exemplary embodiment, one filter element is seated on one face-side end of the filter housing by means of a holding device, and the outer side of the filter element forms an extension of the outer side of the filter housing. If this holding device allows detachment of one filter element from the filter housing, it thus becomes easily possible to clean one filter element detached from the filter apparatus or to replace one filter element with a new one. Here a detachable holding device can be a ring which has been inserted as an adapter between one filter element and the filter housing and which on its outer circumference has peripherally a retaining strip which in the installed state extends both in the direction of one filter element and also in the direction of the filter housing and which, peripherally encompassing the end region of the filter housing facing the ring and also the end region of one filter element facing the ring, makes it possible to hold one filter element which has been placed on the face-side end of the filter housing. In this way, it is easy to change the ion-exchange resin which has been inserted into the filter housing in which one filter element is separated from the filter housing via the holding device, and in this way ion-exchange resin can be removed from the filter housing or can be reinserted into it.

In another exemplary embodiment, the further filter element located on the other face side of the filter housing projects into the latter, and the filter bed encompasses the further filter element so that at a high packing density the filter bed can be accommodated in the filter housing in a space-saving manner.

In that the filter fineness of the further filter element is finer than the particle size in the mixture of the ion-exchange resins, it is guaranteed that unwanted discharge of the ion-exchange resins from the filter apparatus is avoided so that the cleaned fluid leaving the filter apparatus is not contaminated by particles of the ion-exchange resin either. In that the filter bed at least partially encompasses the further filter element as shown, furthermore a large-area, low-impediment incident flow of the fluid which has passed through the filter bed against the further filter element is enabled for removing the last particulate dirt by the further filter element designed as fine filter before the fluid leaves the filter apparatus.

In one advantageous exemplary embodiment, the further filter element is supported by a base part which with its foot part is made as an end cap for the filter housing and, arranged with its head part coaxially to the longitudinal axis of the filter housing, has another passage opening for the exit of the fluid which has been altogether cleaned by means of the filter apparatus. In that the further filter element is supported by a type of end cap of the filter housing, mounting and optionally replacement of the further filter element of the filter apparatus are simple even in this end region.

In one preferred embodiment of the filter apparatus according to the invention, it is provided that, on its end opposite the base part, the further filter element is sealed with an end cap on which parts of the filter element can be supported in the manner of a column. Preferably, here it is furthermore provided that the diameter of a first column which is supported by the end cap of the further filter element be smaller than the passage opening of the holding device and that another column which is arranged concentrically to the first column being supported on the base part of the filter housing stabilizes the first column. If the columns have a different density for the ion-exchange resin of the filter bed, both the trickle speed and also the trickle flow characteristic within the filter bed can be set. Thus, for example, the other column which is located outermost, made as a hollow column, can support in the compressed frame the solidly made column of lower density which is innermost and can induce an intensified flow characteristic in the direction of the center of the filter apparatus. In the opposite case, there is the possibility of routing fluid medium out of the center to the outer side of the device in order in this way, in spite of the central fluid feed via the upper filter apparatus, to force a uniform fluid penetration of the filter bed. It is also possible to fill the different columns with different contents (percent by amount or volume) of anionic and cationic ion-exchange resin in order in this way to optimize the process control for filter bed cleaning. In addition to two columns which are arranged concentrically to one another, it is also possible to arrange several columns concentrically within the filter apparatus in the manner of a “column guide.”

Furthermore, it is preferably provided that the active volumes of two columns being used for fluid cleaning be essentially of the same size, and the filter bed, viewed in the installed state of the filter apparatus, is designed to form above the further filter element a closed cylindrical function block in order in turn to achieve centered fluid guidance within the device with a relatively long residence time of the fluid.

In one advantageous exemplary embodiment, the filter apparatus is connected in the bypass flow of a hydraulic system and is located below the fluid level of the hydraulic system to prevent air from accumulating in the fluid. By placing the filter apparatus in the bypass flow of the hydraulic system, a matched volumetric flow velocity and traverse flow velocity of the fluid through the filter apparatus can be adjusted so that it can be operated at its optimum operating points. An accumulation of air in the fluid should be avoided, however, so that the filter apparatus should accordingly be located below the fluid level of the hydraulic system. At the same time, however, the attempt is also generally made to keep the height difference between the filter apparatus and the fluid level as small as possible in order to effectively prevent the inflow of contaminants which are found with greater frequency in the vicinity of the bottom of the tank, which is intended for the fluid, into one filter element of the filter apparatus.

In one advantageous exemplary embodiment, the anionic ion-exchange resin and the cationic ion-exchange resin, each being present in the form of a granulate, mixed with one another, form the filter bed. In another exemplary embodiment, the filter bed consists of 60% to 95% anionic ion-exchange resin and 40% to 5% cationic ion-exchange resin, preferably of 70% to 90% anionic ion-exchange resin, and of 30% to 10% cationic ion-exchange resin. However, it is especially preferred that the ion-exchange resin consist of 90% anionic ion-exchange resin and 10% cationic ion-exchange resin.

The invention is detailed below using one exemplary embodiment shown in the drawings. The single FIGURE shows a longitudinal section of one exemplary embodiment of the filter apparatus according to the invention.

The FIGURE shows a longitudinal section of one exemplary embodiment of a filter apparatus 10 according to the invention for cleaning of functional fluids, having a filter housing 12 in which a filter bed 14 is placed which preferably consists of a mixture of anionic and cationic ion-exchange resins. Furthermore, the filter apparatus 10 has a filter element 16 which, designed as a flow guide device, allows a diversion of the fluid from a radial into an axial flow direction 18. The respective flow direction 18 is shown in FIG. 1 by an arrow which is oriented accordingly along the approximate fluid flow path 20, which is shown by a broken line, outside and inside the filter apparatus.

Viewed in the flow direction 18 of the fluid, one filter element 16 is located in front of the filter bed 14, i.e., upstream of the filter bed 14, while another flow guide device, made as filter element 22, is located following the filter bed 14, i.e., downstream of the filter bed 14.

The filter housing 12, which is made as a cylinder with a circular base area, has a longitudinal axis 24 and consists preferably of a metallic material or of plastic and is made as a pipe body which in its hollow interior makes available a receiving space for the ion-exchange resin 14 which is to be added.

One filter element 16 which is made in the form of a circular cylinder and which is exposed to incident flow by the fluid from outside the filter apparatus 10 in a radial flow direction 18 has, on its outer jacket facing the fluid, a peripheral filter mat 26 which is present preferably in pleated form. On its inner side, the filter mat 26 is supported by a support pipe 28 through which the fluid can penetrate into the interior of one filter element 16. The support pipe 28 has roughly the same longitudinal extension in the direction of the longitudinal axis 24 as the filter mat 26 so that there is one planar contact surface on each of the two face sides of one filter element 16. The support pipe 28 consists, for example, of a metallic material or a suitable plastic material. The flow path 20 of the fluid which has been pre-cleaned by one filter element 16 continues parallel to the longitudinal axis 24 when leaving one filter element 16 in the axial flow direction 18 in which the fluid travels to the filter bed 14 when the filter apparatus 10 is being used.

One filter element 16 is seated on the face-side end of the filter housing 12, which is the upper end viewed looking at FIG. 1, by means of a holding device 30, and the outer side 32 of one filter element 16 forms an extension of the outer side 34 of the filter housing 12. The holding device 30 is made in the form of an annular adapter which has a roughly H-shaped cross-sectional profile and is aligned coaxially to the longitudinal axis 24. Looking at the installation position of the filter apparatus 10 shown in FIG. 1, the holding device 30 is located above the filter bed 14 and the filter housing 12. In that the outer side 32 of one filter element 16 and the outer side 34 of the filter housing 12 are flush with one another, a flat contact of the radially outer leg 36 of the H-shaped profile of the holding device 30 is ensured both with the outer side 34 of the filter housing 12 and also with the outer side 32 of the one filter element. The upper, radially inner leg 38 of the H-shaped profile of the holding device 30, viewed looking at FIG. 1, projects up into the interior of one filter element 16 and can optionally be moved into contact with the inner side of the support pipe 28. The other inner leg 38 projects down into the interior of the filter housing 12.

The cavity encompassed by the inner legs 38 of the H-shaped profile of the holding device 30 forms a passage opening 40 for the fluid from which particulate dirt has been removed by means of one filter element 16, and the opening is located coaxially to the longitudinal axis 24 of the filter housing 12. In this case, the inner legs 38 of the holding device 30 are located above the level 42 of the inserted filter bed 14. Preferably, there is a cement bond for connecting the holding device 30 to the filter housing 12 and one filter element 16, but exemplary embodiments are also possible which implement a connection which can be detachably fixed, for example, in the form of a screw union or a clamp connection. For a cement bond, it can be provided that a cement layer be applied between the upper face-side end of the filter housing 12 and a web 44 as well as the outer leg 36 of the H-shaped profile of the holding device 30, which leg is the lower one viewed in the direction of looking at FIG. 1, and the outer side 34 of the filter housing 12 in order to cement the filter housing 12 both flush against the web 44 and also, along its outer periphery, flat with the lower outer leg 36 of the holding device 30. To connect one filter element 16 to the holding device 30, it can be provided that the cement material be applied in a peripheral receiving space of the holding device 30, which space extends above the web 14 of the H-shaped profile and is bordered by an outer leg 36 and an inner leg 38.

On its end opposite the holding device 30, one filter element 16 and thus the filter apparatus 10 are terminated by an upper end cap 46 which is made as a flow guide device and whose part that projects into one filter element 16 has an essentially trapezoidal cross section and is made with a peripheral sloped surface 48. One filter element 16 is connected to the upper end cap 46 by an adapter ring 54, which has a roughly U-shaped cross-sectional form that is opened downward looking at FIG. 1 and in which cross-sectional form an upper section of one filter element 16 is held. In this case, an outer ring 52 formed by the radially outside leg of the U-shaped cross section comes into contact with the outer side 32 of one filter element 16, while the radially inner leg of the U-shaped profile is moved into contact with the inner side of the support pipe 28. The upper end cap 46 can be connected detachably or undetachably to the adapter ring 54 via a locking connection or cement bond. Above the adapter ring 54, the upper end cap is provided with a short pipe section 56 which has slots 58 which run in the direction of the longitudinal axis 24. The elasticity of the pipe section 56 achieved by means of the slots 58 allows the filter apparatus 10 to be easily inserted into a suitable receiver (not shown) without tools. Furthermore, a handle 60, for example, in the form of an grip, can be placed around the articulation sites 50 located facing one another in order in this way to be able to easily handle the filter apparatus 10. The handle 60, when it is not needed, can be fixed on catch projections 62 of the adapter ring 54. For connecting one filter element 16 to the adapter ring 54, there can be a cement bond or a detachable connection, for example, in the form of a screw union.

The further filter element 22 is located in the part of the filter apparatus 10 which is the lower one viewed looking at the FIGURE and projects into the filter housing 12. The filter bed 14 comprises the other cylindrical filter element 22 which is aligned around the longitudinal axis 24. A preferably pleated filter mat 64 of the filter element 22 on the outer side surrounds a support pipe 66. The filter fineness of the filter mat 64 is chosen here such that it is finer than the particle size of the mixture of ion-exchange resins of the filter bed 14.

The longitudinal extension of the further filter element 22 in the direction of the longitudinal axis 24 corresponds roughly to the longitudinal extension of one filter element 16 on the opposite end of the filter apparatus.

The further filter element 22 is supported by a base part 68 which is made with its foot part 70 as an end cap for the filter housing 12 and, with its head part 72, which is located in coaxial arrangement to the longitudinal axis 24 of the filter housing 12, has another passage opening 74 for exit of the fluid which has been altogether cleaned by means of the filter apparatus 10.

The further filter element 22 on its end opposite a base part 68 is sealed with an end cap 80 which in its edge region has a peripheral U-shaped cross-sectional profile which is open to the bottom and whose radially inner leg adjoins the inner wall of the support pipe 66 and whose radially outer leg adjoins the outer wall of the filter 64 of the further filter element 22. On the end of the further filter element 22 opposite the end cap 80, the former with its support pipe 66 is put over the head part 72 which projects beyond the base part 68 in the manner of a fitting, while, with the outer side of the filter mat 64, it adjoins a peripheral web 78 which is located on the base part 68 spaced radially from the head part 72. There is a cement bond both for connecting the end cap 80 and also the base part 68 to the further filter element 22, but detachable connections, for example, in the form of screw unions, can also be implemented.

The flow path 20 of the fluid leads first in the axial flow direction 18 out of the filter bed 14 into the radially outer region of the further filter element 22 into which the fluid then flows in radial flow direction 18 in order to finally leave the further filter element 22 and thus the filter apparatus again in the axial flow direction 18 through the other passage opening 74. The foot part 70 of the base part 68 has another peripheral outer ring 76 with which the foot part 70 is in contact with the outer side 34 of the filter housing 12 and which can be connected, for example, by a cement bond. The base part 68, made preferably in one piece, thus terminates the filter apparatus 10, aside from the intended passage opening 74, forming a seal downward.

On the end cap 80 which closes the further filter element 22, parts of the filter bed 14 in the form of a column are supported in the manner of a first column, which column, viewed looking at the FIGURE, projects above the end cap 80 as far as the level 42 of the filter bed 14. In this connection, the diameter of the first column, which is supported by the end cap 80, is smaller than the diameter of the passage opening 40 of the holding device 30. A further column of the filter bed which is located concentrically to the first column and which is supported on the base part 68 of the filter housing 12 stabilizes the first column, which, for its part, is supported on the end cap 80 in the axial direction. In the installed state of the filter apparatus 10 shown in the FIGURE, the filter bed 14 above the further filter element 22 forms a closed cylindrical function block whose installation size, viewed in the direction of the longitudinal axis 24 of the filter apparatus, is larger than the correspondingly viewed installation size of the further filter element 22. In particular, it can be provided that the longitudinal extension of the cylindrical function block corresponds to two to ten times the corresponding installation size of the further filter element 22. The indicated columns are not detailed in the FIGURE and result from imaginary extensions above the end cap 80 and the support surface of the base part 68 which has remained free.

According to the illustrated approximated flow path 20, in the normal operating state the fluid to be cleaned flows first in radial flow direction 18 through the first filter element 16 in order in this way to be partially cleaned. At least partially diverted by the sloped surface 48 of the upper cap 46, the fluid on the further flow path 20 in the axial flow direction 18 flows through the filter bed 14, which forms a type of trickle bed. Then the fluid is diverted into radial flow direction 18 and flows through the further filter element 22 for extremely fine filtration. The fluid which has been cleaned in this way leaves the filter apparatus 10 in the axial flow direction 18.

When installed in a hydraulic system (not shown), it can be provided that, in order to prevent an accumulation of air in the fluid, the filter apparatus 10 in its operating position shown in FIG. 1 be located with its one filter element 16 below the fluid level 82 of the fluid which is to be cleaned.

Let it be further noted that the filter apparatus 10 is suitable not only for removing the indicated contaminants, such as acids, metallic soaps, and basic media, but also, for example, for removing long-chain alcohols. 

1. A filter apparatus (10) for cleaning of functional fluids, especially chosen from among phosphate esters, phosphate-ester based oils, mineral oils, and phenols, having a filter housing (12) into which a filter bed (14) is inserted, especially consisting of a mixture of anionic and cationic ion-exchange resins, and having at least one filter element (16) which, designed as flow guide device, allows a diversion of the fluid from a radial into an axial flow direction (18) or vice versa.
 2. The filter apparatus (10) according to claim 1, characterized in that, viewed in the flow direction (18) of the fluid, at least one flow guide device made as filter element (16) is located upstream of the filter bed (14) and another flow guide device made as filter element (22) is located downstream of the filter bed (14).
 3. The filter apparatus (10) according to claim 1, characterized in that one filter element (16) is seated on one face-side end of the filter housing (12) by means of a holding device (30), and that the outer side (32) of one filter element (16) forms an extension of the outer side (34) of the filter housing.
 4. The filter apparatus (10) according to claim 3, characterized in that the further filter element (22), located on the other face side of the filter housing (12), projects into the latter and that the filter bed (14) at least partially encompasses the further filter element (22) which has a filter fineness that is finer than the particle size in the mixture of the ion-exchange resins of the filter bed (14).
 5. The filter apparatus (10) according to claim 3, characterized in that the holding device (30) in the installed state of the filter apparatus (10) is located above the filter bed (14) and has a through opening (40) for the fluid from which the particulate dirt has been removed by means of one filter element (16), and which opening is located coaxially to the longitudinal axis (24) of the filter housing (12).
 6. The filter apparatus (10) according to claim 2, characterized in that the further filter element (22) is supported by a base part (68) which with its foot part (70) is made as an end cap for the filter housing (12) and, located with its head part (72) in coaxial arrangement to the longitudinal axis (24) of the filter housing (12), has another passage opening (74) for the exit of the fluid which has been altogether cleaned by means of the filter apparatus (10).
 7. The filter apparatus (10) according to claim 6, characterized in that the further filter element (22) is sealed on its end opposite the base part (68) with an end cap (80) on which parts of the filter bed (14) are supported in the form of a first column.
 8. The filter apparatus (10) according to claim 7, characterized in that the diameter of a first column supported by the end cap (80) of the further filter element (22) is smaller than the passage opening (40) of the holding device (30) and that another column, which is arranged concentrically to the first column being supported on the base part (68) of the filter housing (12), stabilizes the first column.
 9. The filter apparatus (10) according to claim 8, characterized in that the active volumes of the two columns for fluid cleaning are of essentially the same size.
 10. The filter apparatus (10) according to claim 2, characterized in that, viewed in the installed state of the filter apparatus (10), the filter bed (14) above the further filter element (22) forms a closed cylindrical function block whose installation size, viewed in the axial longitudinal direction of the filter apparatus, is greater than the correspondingly viewed installation size of the further filter element (22).
 11. The filter apparatus (10) according to claim 2, characterized in that in the normal operating state, the fluid to be cleaned flows first in the radial flow direction (18) through the first filter element (16), in the axial flow direction (18) partially cleaned, and flows through the filter bed (14), which forms a kind of trickle bed with the ion-exchange resins in order to then again flow through the further filter element (22) in the radial flow direction (18), diverted for extremely fine filtration, and that the fluid which has been cleaned in this way leaves the filter apparatus (10) in the axial flow direction (18).
 12. The filter apparatus (10) according to claim 1, characterized in that the filter apparatus (10) is connected in the bypass flow of a hydraulic system and the filter apparatus (10) is located below the fluid level (82) of the hydraulic system to prevent air from accumulating in the fluid.
 13. The filter apparatus (10) according to claim 1, characterized in that the anionic ion-exchange resin and the cationic ion-exchange resin, each present in the form of a granulate, mixed with one another, form the filter bed (14).
 14. The filter apparatus (10) according to claim 13, characterized in that the filter bed (14) consists of 60% to 95% anionic ion-exchange resin and of 40% to 5% cationic ion-exchange resin, preferably of 70% to 90% anionic ion-exchange resin, and of 30% to 10% cationic ion-exchange resin, especially preferably of 90% anionic ion-exchange resin and 10% cationic ion-exchange resin. 